Typically, radio frequency (RF) transmitters, such as those used in radiotelephones, employ standard phase modulation (PM) techniques. PM transmitters employ power amplifiers (PAs) that are highly efficient and have large amplitude nonlinearity because there is no amplitude modulation (AM) component in the modulation format. These PAs often employ transistor bias techniques whereby the bias current is minimized and the output power is controlled by changing the bias current.
Recently, there has been a need to have a transmitter for transmitting signals having AM components and PM components. An AM/PM transmitter requires modulation circuitry for amplitude and phase. Typically, the modulation circuitry is accomplished with control loop circuit configurations. FIG. 1 is a detailed block diagram of a prior art AM/PM transmitter having an AM 115 control loop and a PM 117 control loop. In addition to the phase modulation, the PM control loop 117 performs a frequency translation of a phase reference signal 121 to the desired output frequency. The PM control loop 117 includes a mixer 101, a phase detector 103, and a voltage controlled oscillator (VCO) 105. The mixer generates an intermediate frequency signal 127 having a frequency which is equal to the difference between the frequency of a frequency reference input signal 123 and the frequency of the signal which is fed back from the VCO 105. The phase comparator 103 generates an error signal based on the difference in phase of an intermediate frequency signal 127 and a phase reference input signal 121. The phase comparator output drives a tuning port of the VCO such that the VCO output signal has a phase which is approximately equal to the phase of the phase reference input signal 121, thus providing the phase modulation of the VCO output signal. To perform the frequency translation, the phase comparator output drives the VCO tuning port such that the VCO output signal has a frequency which is equal to the frequency of the local oscillator input signal plus the frequency of the phase reference signal.
The AM control loop is integrated about a power amplifier (PA) 107 having an output amplitude control port. The AM control loop 115 includes the PA 107, an output power coupler 109, an envelope detector 111 and a difference amplifier 113. A portion of the PA output signal is fed back to the envelope detector 111 via the coupler 109. The difference amplifier 113 generates an error signal based on the difference in voltage of the envelope detector 111 output and the amplitude reference input signal 125. The difference amplifier 113 drives the PA output amplitude control port such that the amplitude of the PA output is responsive to the voltage of the amplitude reference input signal 125, thereby achieving amplitude modulation of the power amplifier output signal.
A problem arises when the AM control signal going to the PA control port causes a phase shift on the PA output signal, referred to as an AM to PM conversion effect. This AM to PM conversion effect is a result of a nonlinearity of the PA that is characteristic of PA's which employ design techniques used to minimize bias current and maximize power efficiency, and when the output power is controlled by varying the bias current.
In the past, several approaches have been used to eliminate this AM to PM conversion effect. First, by using a more linear PA module, AM to PM effects were reduced. However, a linear PA is inefficient and power-consuming, and is not desirable for applications such as portable radio telephones. Second, the phase of the reference signal was adjusted such that the PA input signal phase is predistorted thereby canceling the phase distortion which occurs in the PA. However, the required degree of predistortion is dependent on output signal level, supply voltage, and temperature resulting in a very complex open loop control scheme. Third, the PA was embedded within a predistortion loop which shifted the phase of the PA input signal such that the total phase shift through the predistortion loop was automatically forced to zero.
FIG. 2 is a simplified block diagram of a PA embedded in a predistortion loop. In the circuit, the input signal passes through an input coupler 201 and a phase shifter 203 before going into the PA 205. The PA output signal passes through an output coupler 207. The input coupler 201 and output coupler 207 feed portions of the input signal and the PA output signal to the phase comparator 209. The phase comparator generates an error signal based on the difference in phase between the input signal and the PA output signal. The phase comparator 209 output drives the control port of the phase shifter 203 such that the PA input is automatically predistorted and the PA output signal has a phase which is approximately equal to the input signal. The predistortion loop thereby eliminates the phase distortion in the PA. However, the predistortion loop circuitry adds unwanted cost and complexity. Furthermore, a problem arises when the phase shift of the PA exceeds the finite range of the phase shifter.
Thus, it would be desirable to develop an AM/PM transmitter capable of frequency modulation, frequency translation and amplitude modulation, wherein there are no restrictions on the phase distortion which occurs in the PA, nor dependency upon the output signal level, the supply voltage, or the temperature.